1. Field of the Invention
This invention relates generally to a system for supplying a lubricant and more particularly to a system for providing lubricant to a bearing assembly. Even more particularly, the bearing assembly may be within a control moment gyroscope or a reaction wheel assembly for a spacecraft.
2. The Prior Art
The appropriate provision of lubricant to any bearing assembly is important, if not critical, to the life and proper operation of the bearing. Proper lubrication is even more critical in certain environments. For example, in order to maintain the low drag torque performance of spin bearings for operation in space, it is necessary to use very limited quantities of oil or grease. The life of such bearing systems is limited by the reduction in quantity of and degradation of the lubricant. In order to extend the useful life of such a bearing and lubrication system, an active oil lubrication system was developed, as disclosed in U.S. Pat. No. 4,738,336.
The system disclosed in U.S. Pat. No. 4,738,336 included three primary components and was designed for a large spin bearing application. The bearing and lubrication system were comprised of a spring loaded bellows reservoir, which was mounted in a housing and connected through a solenoid activated valve and a length of tubing to the bearing. When commanded, the valve opened for a specific period of time and a quantity of oil was delivered to the bearing. The system also included a sensor to detect the presence of an adequate amount of lubricant in the bearing and control system to determine when additional oil was required. In that system, the exact quantity of lubricant was a function of the amount of time that the valve was opened, the temperature of the lubricant, since that affects the viscosity, and the amount of lubricant that had already been delivered, since the reservoir pressure was reduced as oil was depleted.
Although this system provided many advantages, it does not lend itself to spacecraft applications in which weight and envelope are critical parameters. Additionally, the reservoir pressure can vary over the useful life of the device.
In order to improve upon the system disclosed in U.S. Pat. No. 4,738,336, another system was developed in an effort to maintain the oil delivery quantity more constant over the life of the system. This improved system is disclosed in a presentation entitled "Positive Lubrication System," which was presented at the 24th Aerospace Mechanism Symposium, Apr. 18-19, 1990, at the NASA-Kennedy Space Center.
In the improved system disclosed in "Positive Lubrication System," an accumulator reservoir was provided and was supplied with lubricant from the main oil reservoir. A tubing assembly, which had a high flow resistance, provided the delivery of lubricant to the bearing and assured that the flow of oil was initiated only after the accumulator had been fully charged. Even though this system provided additional advantages it, too, because of its size, does not lend itself to bearing lubrication applications in densely packaged spacecraft components.